Mineral grinding aids and process of grinding



United States Patent C) 3,325,105 MINERAL GRINDING AIDS AND PROCESS OF GRINDING Preston L. Veltman, Severna Park, Md., assignor to W. R.

Grace & Co., Cambridge, Mass, a corporation of Connecticut No Drawing. Filed July 17, 1964, Ser. No. 383,519

5 Claims. (Cl. 241-16) This invention relates to grinding minerals. More particularly, it relates to the use of an additive for improving the grinding and pack set characteristics of minerals, particularly Portland cement.

The term mineral as used herein is intended to include minerals and mineral compositions as well as mixtures of minerals and partially processed minerals. As examples of materials contemplated by the term mineral, mention may be made of iron ore, phosphate rock, bauxite, fluxed and unfluxed aluminum oxide, cement clinker, and beryl.

During the grinding of minerals, cleavage of the particles exposes fresh or nascent surfaces which have high energies due, probably, to the breaking of ionic bonds. The surface forces of the particles persist for some time after grinding and lead to pack set and/or poor fluidity if such forces are not reduced. Undue reduction or the complete absence of surface forces or attraction between the mineral particles is undesirable because the finely ground mineral becomes excessively fluid. On the other hand, if the surface forces are too great, the mineral tends to pack set and has a low bulk density because of the presence of high void volume agglomerates. Low bulk densities mean that less mineral by Weight can be stored in a hopper or silo.

It has been known that certain polar molecules when added to certain minerals either during or after grinding will attach to the mineral particles and reduce their surface forces. If the amount of additive used or if the time of addition is not correct, however, optimum dry dispersion is not obtained. For example, if an unduly active additive is present during grinding, it will cause a mineral to have an extreme fluidity while in the mill. This causes the mineral to pass rapidly through the mill before it is reduced to the desired size, and consequently the number of recycle steps of oversized material is increased. Conversely, an additive which produces the desired degree of dry dispersion when ground with the mineral may not be realized if admixed after grinding.

Pack set inhibition refers to a reduction in the amount of energy necessary to initiate flow in the ground mineral. Most minerals when compacted by vibration become semirigid and will not fiow until considerable mechanical effort has been applied to disintegrate the semirigid mass. The pack set inhibition property is particularly important in unloading the finely ground mineral from storage silos after transportation in trucks, barges, and hopper cars.

A novel additive has now been found which serves as both a grinding aid and pack set inhibitor. The novel additive of the present invention comprises an amino acid, a salt of an amino acid, or an ester of amino acid. This additive is interground with the mineral in the grinding mill to provide increased grinding efiiciency as well as other advantageous results.

As examples of amino acids suitable for use in the present invention mention may be made of the following:

Patented June 13, 1967 CH3SH Cysteine NHr-CH-C O OH CHr-CHrSCHa Methionine N C'HzC O ONa CHr-C O O Na.

NHz-CH2-C OK NHzCH2CHz-CH2C O O Na NHr-CHz-CHr-CHzCHz-C OONa NHrC 4- C O O Na CHz-CH-C H3 CH4 As examples of suitable esters of amino acids, mention may be made of the following:

N Hz-CH-C O O H CHzNH Methyl a,fi-diamino-propionate NHgCHz-C O 0 Q2115 Glycine ethyl ester CHz-CHz-CH;

While the novel additive of the present invention is applicable, as stated above, to all minerals, e.g., iron ore, bauxite, phosphate rock, beryl and cement, for convenience, the use of the additive of the present invention will be described in terms of a grinding aid for cement, and more particularly Portland cement.

Portland cement represents a class of hydraulic cements and is comprised essentially of two calcium silicates and a lesser amount of calcium aluminate. These cements are produced by heating an intimate mixture of finely divided calcareous material (limestone) and argillaceous material (clay) to fusion to form a clinker. The clinker is ground with the addition of about 2 percent gypsum, or some other form of calcium sulfate, to obtain the desired setting qualities in the finished cement. It is to the clinker that the novel additive of this invention is preferably added to increase grinding efliciency and to inhibit subsequent pack set in the finished cement.

The grinding aid of this invention may be utilized over a fairly wide range. It is generally added at the level of about 0.005 to 1.0 percent by weight based on the mill charge of the cement. Preferably, it is used at about 0.009 to 0.015 percent by weight and more preferably, at 0.01 percent by weight. While the additives may be used over a relatively wide range without deleterious effects, it is desirable to avoid the use of certain amino acids in the grinding of cement at relatively high levels because of the possibility of air entrainment in the concrete, mortar or neat paste. As an example of such amino acids, mention may be made of acids which contain double bonds.

Alternatively, the additive may be combined with the limestone and clay in the formation of the clinker, rather than being added separately to the already formed clinker in the mill.

The additive reduces the surface forces of the cement to a desired degree of dry dispersion that results in improved grinding and mill retention time as well as facilitating storage in transportation. The action of the additive prevents reagglomeration of the cement particles and increases the efficiency of the grinding operation.

Amino acids may be divided into three classes: neutral, basic, and acidic amino acids. The neutral amino acids have an equal amount of amino and carboxyl groups; the basic amino acids have an excess of amino groups to carboxyl groups; and the acidic amino acids have an excess of carboxyl groups to amino groups. It is the acidic amino acids, especially glutamic acid and nitrilotriacetic acid, which are preferred.

The preferred esters are the methyl and ethyl esters, e.g., glycine ethyl ester and glycylglycine ester.

The preferred salts of amino acids are the sodium, potassium and calcium salts, e.g., the sodium salt of nitrilotriacetic acid and the potassium salt of glycine.

The additive may be utilized in either the dry form or in aqueous solution. For ease of addition, since the additive is used in such a relatively small amount, an aqueous solution is preferred.

The effectiveness of the additive as a grinding aid is illustrated in the following example. The expression percent by weight solids additive refers to the active ingredient irrespective of whether an aqueous solution or not.

Example Type 1 Portland cement clinker containing 5 percent gypsum was charged to a laboratory steel ball grinding mill. The mill was closed and heated to a temperature between 210 F. and 230 F. prior to rotation. 0.05 percent by weight solids based on the solids of clinker of nitrilotriacetic acid in 60 percent aqueous solution was then added to the mill. After a number of revolutions the mill was stopped, and the properties of the ground cement were determined. The table below shows the results of the cement prepared in the above example. An untreated cement, i.e., without any grinding aid and a cement which has been interground with a commercial grinding aid (0.01 percent by weight solids additive) are included for purposes of comparison.

It will be noted from the above table that the use of the additive provided substantially the same surface area as the cement without a grinding aid with about 13 percent less grinding as measured by revolutions of the mill. The additive provided a cement with about the same surface area as a commercial grinding aid with about 9 percent less grinding. Obtaining approximately the same surface area with a shortened grinding time indicates that improved grinding efiiciency is achieved With the use of the additive of this invention.

Similar results are obtained when other minerals and mineral compositions are ground with the additive of this invention. The additive is generally used in the same ratio to all minerals as that described above in connection with the cement, i.e. 0.005 to 1.0 percent solids on solids, preferably 0.009 to 0.015 percent and more preferably 0.01 percent. It should be understood that all minerals will not be processed in the same manner as cement prior to grinding, i.e., the formation of the clinker. Some minerals may not be processed at all, e.g., phosphate rock, while some may be given some degree of processing prior to grinding.

What is claimed is:

1. In a process for grinding iron ore, the steps which 6 comprise adding to the mineral mill charge a compound selected from the group consisting of amino acids, esters of amino acids, and salts of amino acids, and grinding the mineral.

2. The process as defined in claim 1 wherein said amino acid is nitn'lotriacetic acid.

3. The process as defined in claim 1 wherein said amino acid is glutamic acid.

4. A composition comprising iron ore and a compound selected from the group consisting of amino acids, esters of amino acids and salts of amino acids.

5. A composition as defined in claim 4 wherein said amino acid is nitrilotriacetic acid.

References Cited UNITED STATES PATENTS 2,307,696 1/ 1943 Blair-McGufiie 106-91 X 2,499,445 3/ 1950 Arnmann 1O69O 2,900,265 8/1959 Klein 10691 WILLIAM W. DYER, JR., Primary Examiner.

R. J. ZLOTNIK, Assistant Examiner. 

4. A COMPOSITION COMPRISING IRON ORE AND AN COMPOUND SELECTED FROM THE GROUP CONSISTING OF AMINO ACIDS, ESTERS OF AMINO ACIDS AND SALTS OF AMINO ACIDS. 